If a main genre is rock,pop,hip-hop, then emo is a sub-genre of rock, dance of pop, and rap of hip-hop.
Basically, a sub-genre contains elements of another genre, but can't be called that genre. It however, isn't a genre in its own right.

a) Emo is a whole new genre!!
b)Nay, it is but a subgenre
a)Bwuh!?
b)Read this! *directs to this article*

Generally used whenever something computer-related dies, e.g a computer crashes, a site fails, or a conversation dies. Used like a funeral speech: "ashes to ashes, dust to dust," to parody the death of the object involved.